Even though 70 percent patients dying with breast cancer have osteolytic bone metastasis, a thorough understanding of the biology of skeletal metastasis remains elusive due to the lack of appropriate animal models that mimic the clinical experience of metastasis of primary breast tumors to the bone. The mouse has been used as a model for a number of individual steps in the metastatic cascade, but a mouse model for the entire process i.e. development of the primary tumor in the breast to its metastases to distant organs like bone, has not been developed. Although, some transgenic mouse models of breast cancer have been shown to metastasize extensively, none of them metastasize to the bone. Overexpression and amplification of the neu (c-erbB2, ERBB2) proto-oncogene have been implicated in the development of aggressive human breast cancer. Targeted expression of the neu proto-oncogene specifically to the mouse mammary gland (under the control of a MMTV promotor) induces metastatic disease after a mean latency of 205 days. On the other hand, it is generally recognized that ostoclastic bone resorption is necessary before expansion of tumor cells from bone marrow to bone. In the case of breast cancer, this final step is mediated by osteoclasts that are stimulated by local production of the tumor peptide parathyroid hormone - related peptide (PTH-rp). Some of the evidence which supports this hypothesis are: patients with PTH-rp positive breast carcinomas are more likely to develop bone metastasis. In addition, breast carcinoma metastatic to bone express PTH-rp in greater than 90 percent of cases, compared with only 17 percent of metastases to nonbone sites. It has also been reported that expression of PTHrp in MDA 231 human breast cancer cells increased osteolytic bone metastasis while neutralising antibodies to PTHrp markedly suppressed them. Thus, PTHrp plays an important role in the process of bone metastasis. Taken together, both the clinical and experimental data suggest that if neu-expressing breast carcinoma cells are engineered to overexpress PTHrp, they may be able to home to the bone and metastasize. With this hypothesis, the overall objective of this proposal is to develop a stochastic model of breast cancer metastasis to the bone and to identify downstream targets that mediate the activity of ERBB2 and PTHrp during breast cancer metatatic progression. We propose to test this hypothesis both in vitro, using a cell culture model and in vivo by generating neu/PTHrp expressing bigenic mice. Our specific aims are: Specific Aim 1: To develop a mouse model of breast cancer metastasis from the mammary gland to the bone using an in vitro mouse cell culture model that is engineered to overexpress both neu and PTHrp. Specific Aim 2: To determine, if, targeted expression of PTHrp and neu specifically to the mouse mammary epithelial cells facilitates bone metastases formation in these animals. Development of such a model that overexpresses proto-oncogene neu and is engineered to overproduce PTHrp would most closely mimic human breast cancer progresion and provide an improved model for preclinical studies of therapeutic agents. It will also provide a unique platform to investigate cellular events involved in site-specific metastasis of breast cancer to the bone.